Syntheses and crystal structures of new vanadium(IV) oxyphosphates M(VO)2(PO4)2 with M = Co, Ni

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Syntheses and crystal structures of new vanadium(IV) oxyphosphates M(VO) ₂ (PO ₄ ) ₂ with M ¼ Co, Ni

S. Kaoua ^a , P. Gravereau ^b , J.P. Chaminade ^{b , *} , S. Pechev ^b , S. Krimi ^a , A. El Jazouli ^c

a

Laboratoire de Physico Chimie des Mate´riaux Inorganiques, Faculte´ des Sciences Aı¨n Chock, Casablanca, Morocco

b

Institut de Chimie de la Matie`re Condense´e de Bordeaux (ICMCB-CNRS), Universite´ Bordeaux I, 33608 Pessac, France

c

Laboratoire de Chimie des Mate´riaux Solides, Faculte´ des Sciences Ben M’Sik-UH2M, Casablanca, Morocco

a r t i c l e i n f o

Article history:

Received 25 July 2008 Received in revised form 21 November 2008 Accepted 27 November 2008 Available online 13 December 2008 Keywords:

Co(VO)

2

(PO

4

)

2

Ni(VO)

2

(PO

4

)

2

Vanadium(IV) oxyphosphate compound Single crystal and powder X-ray diffraction

a b s t r a c t

We have extended our research interest on titanium oxyphosphates (M

^II

(TiO)

2

(PO

4

)

2

, with M

^II

¼ Mg, Fe, Co, Ni, Cu, Zn) to vanadium oxyphosphates M

^II

(V

^IV

O)

2

(PO

4

)

2

(M

^II

¼ Co, Ni). For each compound two phases, named a and b according to synthesis conditions, have been stabilized at room temperature, then characterized. The four crystal structures M(VO)

2

(PO

4

)

2

( a and b for M¼ Co, Ni) have been determined in monoclinic P2

1

/c space group using X-ray single crystals diffraction data. Structure of the a phase is derived from the Li(TiO)(PO

4

) (orthorhombic Pnma) and LiNi

0.50

(TiO)

2

(PO

4

)

2

(monoclinic P2

1

/c) types, with cell parameters: a ¼ 6.310(1) Å, b ¼7.273(1) Å, c ¼7.432(1) Å, b ¼ 90.43(1)

for M ¼ Co, and a ¼ 6.297(2) Å, b ¼ 7.230(2) Å, c ¼ 7.421(2) Å, b ¼90.36(2)

for M ¼ Ni. Structure of the b phase is derived from the Ni(TiO)

2

(PO

4

)

2

-type (monoclinic P2

1

/c) with cell parameters: a ¼ 7.2742(2) Å, b ¼7.2802(2) Å, c ¼ 7.4550(2) Å, b ¼120.171(2)

for M ¼ Co, and a ¼7.2691(2) Å, b ¼ 7.2366(2) Å, c ¼ 7.4453(2) Å, b ¼ 120.231(2)

for M ¼ Ni. All these structures consist of a three dimensional (3D) framework built up of infinite chains of tilted corner-sharing [VO

6

] octahedra, cross-linked by corner-sharing [PO

4

] tetrahedra.

The M

^2þ

ion (M ¼ Co, Ni) is located in a triangular based antiprism which shares faces with two [VO

6

] octahedra. Structural filiation is discussed based on a common structural unit, a sheet where divalent cations M

^2þ

(M ¼ Co, Ni) are inserted. A thermal study of the a 4 b transition is also presented.

Ó 2008 Elsevier Masson SAS. All rights reserved.

1. Introduction

The research on phosphate materials is currently in progress due to their exceptional optical and laser properties [1,2]. The potas- sium titanyl phosphate KTiOPO4 [3,4] is a well-known material for its nonlinear optical properties which is of technological impor- tance. The synthesis of vanadyl phosphates M(VO)

₂

(PO

₄

)

₂

was undertaken in order to look for new material analogues to KTiOPO4, in which K

^þ

has been replaced by divalent ions M

^2þ

and Ti

^4þ

by V

^4þ

. Two new compounds Co(VO)

₂

(PO

₄

)

₂

and Ni(VO)

2

(PO

4

)

2

were obtained, with respectively two phases named a (low temperature) and b (high temperature). Recently these compounds were announced in a publication devoted to catalysts for selective oxidations of light hydrocarbons [5]. In this paper cell parameters and space group are given and structural relationships are discussed based on Lazulite-types, but structure determinations are not yet published. Here we present methods of syntheses in

both forms, powder and single crystals. Single crystals are obtained by grain growth in the solid phase without chemical vapour transport as used by Glaum et al. [5]. Atomic structural parameters for the four phases, description filiations based on our precedent results on titanyl phosphates [6–9] and a thermal study of the a 4 b transition have been performed.

2. Experimental section

2.1. Powder syntheses

The reagents used for the synthesis of M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni) were VO

₂

(or V

₂

O

₃

and V

₂

O

₅

) and M(PO

₃

)

₂

(M ¼ Co, Ni). M(PO

₃

)

₂

metaphosphates were prepared from stoichiometric proportions of M(NO

3

)

2

$6H

2

O (M ¼ Co, Ni) and NH

4

H

2

PO

4

dissolved in distilled water, then heated progressively until 600

C. Crystalline powders of M(VO)

₂

(PO

₄

)

₂

(M ¼ Co, Ni) were synthesized by solid state reaction, according to the following reaction: M(PO

3

)

2

þ 2VO

2

/ M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni). Stoichiometric proportions of the reagents were put in a gold tube. The gold tube was introduced into silica tube which was sealed under vacuum and heated at 700

C for

* Corresponding author. Tel.: þ33540006265; fax: þ33540002761.

E-mail address: chamin@icmcb-bordeaux.cnrs.fr (J.P. Chaminade).

Contents lists available at ScienceDirect

Solid State Sciences

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s s s c i e

1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.solidstatesciences.2008.11.011

40 h for a phase, and at 900

C for 40 h for b phase. The powders of M(VO)

₂

(PO

₄

)

₂

(M ¼ Co, Ni) are green. Their purity was checked by X-ray diffraction analysis with a ‘‘PANALYTICAL X’PERT PRO’’ q – q - diffractometer (Cu K a

_1,2

radiation). The use of the ‘‘profile match- ing’’ option of FULLPROF program [10] confirms that all the observed diffracted lines are taken into account by the proposed space group, P2

1/c

. In the case of a phase of Ni compound a Rietveld refinement has been done using FULLPROF and a PANALYTICAL X’PERT PRO’’ q –2 q (Cu K a

1

-Ge incident monochromator) diffrac- tometer data. The quality of this refinement (cR

p

¼ 0.113 and

cR

wp

¼ 0.142 (without background); c

²

¼ 2.04; R

b

¼ 0.044 and R

_f

¼ 0.041) constitute an ‘‘a posteriori’’ method for checking chemical composition and structural model. The patterns are different between a and b -ones but are very similar when changing M (Co or Ni). Fig. 1 shows results for Ni compounds.

2.2. Thermal study

DTA investigation was performed with MDTA 85 ‘‘SETARAM’’, 5

C/min.

Fig. 1. Powder X-ray diffraction patterns of Ni(VO)

2

(PO

4

)

2

. a. Powder X-ray diffraction pattern of a -Ni(VO)

2

(PO

4

)

2

. b. Powder X-ray diffraction pattern of b -Ni(VO)

2

(PO

4

)

2

.

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634 629

High-temperature X-ray diffraction study has been done with a ‘‘PANALYTICAL X’PERT MPD’’ q – q -diffractometer (Cu K a

_1,2

radia- tion) with a HTK16 Anton Paar furnace. Platinum heating filament is filled with powder dusted with 20 m m sieve and sample surface is corrected with a razor blade.

2.3. Single crystals and X-ray diffraction

Observation by optical microscope shows that the powders of a - M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni) contain very small single crystals ( w 20 m m – Table 1). For b -M(VO)

2

(PO

4

)

2

(M ¼Co, Ni) single crystals (Table 2) were obtained by heating the corresponding crystalline powders at 1100

C for 2 h until melting, followed by slow cooling at the rate of 5

C h

¹

until 500

C. The furnace was then put off until room temperature.

Taking into account the small size of single crystals for a - Co(VO)

2

(PO

4

)

2

and a -Ni(VO)

2

(PO

4

)

2

, X-ray measurements were done on a RIGAKU R–AXIS RAPID equipped with a rotating Cu anode. For b -Co(VO)

₂

(PO

₄

)

₂

and b -Ni(VO)

₂

(PO

₄

)

₂

compounds data collections have been done with a Nonius Kappa CCD diffractom- eter (Mo K a radiation with a graphite monochromator). Details are summarized in Tables 1 and 2.

3. Structure determinations

The extinction conditions observed for M(VO)

2

(PO

4

)

2

(M ¼ Ni, Co) agree with the space group P2

₁

/c. The data were corrected for Lorentz-polarization effects, and empirical absorption corrections

were carried out using SCALEPACK program [11]. The structures of both crystals were solved with SHELXS-97 program by deconvo- lution of the Patterson function and the heavy-atom method, and refinements with anisotropic displacement parameters for all atoms were done with SHELXL-97 program [12] (Tables 1 and 2).

The final atomic coordinates with the equivalent isotropic displacement parameters are presented in Tables 3 and 4. Selected interatomic distances and angles and bond valence sums are listed in Table 1

Crystal data and structure refinements for a -M(VO)

2

(PO

4

)

2

with M ¼ Co, Ni.

Formula a -Co(VO)

2

(PO

4

)

2

a -Ni(VO)

2

(PO

4

)

2

Formula weight (g) 382.75 382.52

Crystal size ( m m) 25 25 10 25 25 15

Colour Light green Green

Crystal system Monoclinic

Space group P2

1

/c

Temperature (K) 293(2)

a (Å) 6.310(1) 6.297(2)

b (Å) 7.273(2) 7.230(2)

c (Å) 7.432(2) 7.421(2)

b (

) 90.43(2) 90.36(2)

Volume (Å

³

) 341.1(1) 337.8(2)

Z 2

Calculated density (g/cm

³

)

3.727 3.760

Wavelength (Å) 1.5418

Diffractometer RIGAKU R–AXIS RAPID (rotating Cu-anode)

Scan method Image Plate-multi-scans

Absorption coefficient (mm

¹

)

46.4 31.1

F(000) 366 368

q range (

) 6.5–68.0 6.5–71.5

Index ranges 7 h 7, 8 k 8,

8 l 8

7 h 7, 8 k 8, 8 h 9

Reflections collected (I >0 s (I))

1844 1904

Independent reflections (I >0 s (I))

623 [R(int) ¼ 0.058] 645 [R(int) ¼0.040]

596 reflections

with I >2 s (I)

623 reflections with I > 2 s (I)

Absorption correction Empirical multi-scans Refinement method Full-matrix

least-squares on F

²

Data/restraints/parameters 623/0/71 645/0/71

Goodness of fit on F

²

1.123 1.105

Final R indices R

1

[I> 2 s (I)] ¼0.031 R

1

[I >2 s (I)] ¼ 0.0301 wR

2

[I> 0 s (I)] ¼0.080 wR

2

[I >0 s (I)] ¼ 0.081

Extinction coefficient 0.0014(6) 0.0040(8)

Largest diff. peak and hole (e Å

³

)

0.67 (near V) and 0.51 (near O(3))

0.68 (near O

3

) and 0.60 (near O(3))

Table 2

Crystal data and structure refinements for b -M(VO)

2

(PO

4

)

2

with M ¼Co, Ni.

Formula b -Co(VO)

2

(PO

4

)

2

b -Ni(VO)

2

(PO

4

)

2

Formula weight 382.75 382.52

Crystal size ( m m) 80 60 50 85 65 65

Colour Opaque with metallic

brightness

Greenish Crystal system Monoclinic

Space group P2

1

/c

Temperature (K) 293(2)

a (Å) 7.2742(2) 7.2691(2)

b (Å) 7.2802(2) 7.2366(2)

c (Å) 7.4550(2) 7.4453(2)

b (

) 120.171(2) 120.231(2)

Volume (Å

³

) 341.32(2) 338.39(2)

Z 2

Calculated density (g/cm

³

)

3.724 3.754

Wavelength (Å) 0.7107

Diffractometer BRUKER–KAPPA–CCD

Scan method CCD Scans

Absorption coefficient (mm

¹

)

5.60 5.98

F(000) 366 368

q range (

) 3.5–32.0 3.5–32.1

Index ranges 10 h 10, 10 k 10,

11 l 11

10 h 10,

10 k 10, 11 l 11 Reflections collected

(I > 0 s (I))

4294 4357

Independent reflections (I > 0 s (I))

1185 [R(int)¼ 0.026] 1179 [R(int) ¼0.025]

1075 reflections with I > 2 s (I)

1069 reflections

with I >2 s (I)

Absorption correction Empirical SCALEPACK Refinement method Full-matrix

least-squares on F

²

Data/restraints/parameters 1185/0/71 1179/0/71

Goodness of fit on F

²

1.269 1.090

Final R indices R

1

[I >2 s (I)] ¼ 0.023 R

1

[I > 2 s (I)] ¼ 0.019

wR

2

[I >0 s (I)] ¼ 0.090 wR

2

[I > 0 s (I)] ¼ 0.047

Extinction coefficient 0.046(5) 0.025(2)

Largest diff, peak and hole e Å

³

1.04 (near V) and 0.66 (near Co)

0.47 (near O(3)) and 0.52 (near V)

Table 3

Atomic coordinates and equivalent isotropic displacement parameters (Å

²

10

⁴

) for a -M(VO)

2

(PO

4

)

2

with M ¼ Co (first line), Ni (second italic line).

Atom Wyckoff site x y z U(eq)

Co/Ni 2a 0 0 0 77(3)

0 0 0 65(3)

V 4e 0.73831(9) 0.21958(10) 0.32731(9) 57(3)

0.74064(9) 0.21948(9) 0.32690(8) 47(3)

P 4e 0.24957(14) 0.12245(13) 0.37563(14) 53(3)

0.24987(13) 0.12349(13) 0.37382(12) 46(3)

O(1) 4e 0.7594(4) 0.1565(4) 0.1178(4) 101(7)

0.7645(4) 0.1528(4) 0.1174(4) 92(6)

O(2) 4e 0.7781(4) 0.0059(4) 0.7909(4) 84(7)

0.7795(4) 0.0047(4) 0.7930(4) 76(6)

O(3) 4e 0.2625(4) 0.4918(4) 0.0455(4) 108(7)

0.2622(4) 0.4916(4) 0.0441(4) 92(6)

O(4) 4e 0.4405(4) 0.2519(4) 0.8548(4) 85(6)

0.4419(4) 0.2515(4) 0.8537(4) 73(6)

O(5) 4e 0.9479(4) 0.7494(4) 0.1170(4) 76(6)

0.9469(4) 0.7525(4) 0.1176(4) 61(6)

Tables 5 and 6. Bond valences are calculated using the ‘‘Brown method’’

[13]: V

_i

¼ P

j

V

_ij

; V

_ij

¼ exp [(R

_ij

d

_ij

)/b] with b ¼ 0.37 Å; R

_ij

characterized a cation–anion pair (O

²

: 1.774 Å for V

^4þ

, 1.654 Å for Ni

^2þ

, 1.680 Å for Co

^2þ

and 1.617 Å for P

^5þ

); d

ij

is the distance between i and j atoms. The results are in good agreement with the theoretical values for the expected formal oxidation states of Ni

^2þ

, Co

^2þ

, V

^4þ

, P

^5þ

and O

²

ions.

4. Structure descriptions

The structures of M(VO)

₂

(PO

₄

)

₂

(M ¼ Co, Ni), in both forms a and b , consist of a three dimensional 3D-framework built up from [VO

6

] octahedra, triangular based antiprism [MO

6

] and isolated [PO

4

] tetrahedra. The [VO

₆

] octahedra share corners and form infinite chains along the c-axis (Fig. 2). These chains are linked by phos- phate tetrahedra to constitute the 3D-framework. Two successive [VO

6

] octahedra of the same chain are linked via the oxygen O(1) atom, which does not belong to the [PO

4

] tetrahedron. Co

^2þ

and Ni

^2þ

cations are located in a triangular based antiprism sharing faces with two [VO

6

] octahedra.

4.1. a -M(VO)

₂

(PO

₄

)

₂

(M¼ Co, Ni) 4.1.1. VO

6

octahedra

Vanadium atoms are displaced from the centre of the octahedron giving rise to an alternating long (Co: 2.342(3); Ni: 2.349(3) Å) and short (Co: 1.630(3); Ni: 1.636(3) Å) V–O(1) distances while the four remaining V–O bond distances have intermediate values ranging between 1.903(3) and 2.031(3) Å for Co (average value of 1.974 Å); and between 1.905(3) and 2.022(3) Å for Ni (average value of 1.974 Å).The shortest V–V distance is 3.742 Å for M ¼ Co and 3.737 for M ¼ Ni.

4.1.2. MO

6

polyhedra (M ¼ Co, Ni)

Co

^2þ

and Ni

^2þ

cations are located in a triangular based antiprism sharing faces with two [VO

₆

] octahedra. This connection by faces implies short Co–V (2.916 Å) and Ni–V (2.899 Å) distances. M–O distances are ranging from 2.047(3) to 2.095(3) Å (average value:

2.075 Å) for Co, and from 2.020(3) to 2.048(3) Å (average value:

2.044 Å) for Ni. Average values are in good agreement with the sum of the ionic radii of O

²

(CN ¼ 3) and M

^2þ

ions (CN ¼ 6): 2.11 Å (HS) for Co and 2.05 Å for Ni. The shortest Co–Co and Ni–Ni distances are 5.199 Å and 5.180 Å respectively.

4.1.3. PO

4

tetrahedra

[PO

₄

] tetrahedra share oxygen atoms with four [VO

₆

] octahedral groups. O(2) and O(4) belong to the same chain while O(3) and O(5) belong to two different chains. Therefore, one [PO

4

] group connects

three different chains . –V–O(1)–V–O(1)–V– . The [PO

₄

] tetra- hedra are isolated from each others and are rather regular with O–

P–O angles varying from 106.5(2)

to 112.1(2)

for Co, and from 106.4(2)

to 111.9(1)

for Ni. P–O bond distances are ranging between 1.513(3) Å and 1.559(3) Å for Co and between 1.515(3) Å and 1.557(3) Å for Ni.

4.2. b -M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni)

b -M(VO)

₂

(PO

₄

)

₂

(M ¼ Co, Ni) was found to be isostructural with Ni(TiO)

₂

(PO

₄

)

₂

[6].

4.2.1. VO

6

octahedra

Here also vanadium atoms are displaced from the centre of the octahedron giving rise to an alternating long (Co: 2.362(2); Ni:

2.372(1) Å) and short (Co: 1.633(1); Ni: 1.636(1) Å) V–O(1) distances while the four remaining V–O bond distances have intermediate values ranging between 1.903(2) and 2.031(2) Å for Co (average value of 1.977 Å); and between 1.900(1) Å and Table 4

Atomic coordinates and equivalent isotropic displacement parameters (Å

²

10

⁴

) for b -M(VO)

2

(PO

4

)

2

with M ¼ Co (first line), Ni (second italic line).

Atom Wyckoff site x y z U(eq)

Co/Ni 2a 0 0 0 75(2)

0 0 0 57(1)

V 4e 0.73645(5) 0.22110(5) 0.54141(5) 55(1)

0.73893(4) 0.22085(4) 0.54314(4) 45(1)

P 4e 0.25041(8) 0.12163(7) 0.74838(8) 50(2)

0.25066(6) 0.12292(6) 0.74971(6) 41(1)

O(1) 4e 0.7581(2) 0.1541(2) 0.7602(2) 96(3)

0.7636(2) 0.1505(2) 0.7630(2) 82(2)

O(2) 4e 0.7787(3) 0.0049(2) 0.0988(3) 86(3)

0.7806(2) 0.0039(2) 0.0982(2) 72(2)

O(3) 4e 0.4384(3) 0.2493(2) 0.8627(3) 102(3)

0.4406(2) 0.2495(2) 0.8652(2) 92(3)

O(4) 4e 0.2702(3) 0.0064(2) 0.5905(3) 116(4)

0.2691(2) 0.0060(2) 0.5913(2) 104(3)

O(5) 4e 0.0495(3) 0.2536(2) 0.1390(3) 81(3)

0.0510(2) 0.2504(2) 0.1406(2) 70(2)

Table 5

Bond distances (Å), bond valences (BV) and angles (

) for a -M(VO)

2

(PO

4

)

2

.

M ¼ Co M ¼ Ni

Distance (Å) BV Distance (Å) BV

2 M–O(5)

^i,ii

2.047(3) 0.383 2.020(3) 0.372 2 M–O(2)

^iii,iv

2.084(3) 0.347 2.064(3) 0.330 2 M–O(1)

^v,vi

2.095(3) 0.336 2.048(3) 0.345

C2.075D P

s ¼2.13 C2.044D P s ¼2.09

V–O(1) 1.630(3) 1.516 1.636(3) 1.492

V–O(4)

^vii

1.903(3) 0.725 1.905(3) 0.721

V–O(3)

^viii

1.907(3) 0.717 1.905(3) 0.721

V–O(2)

^vii

2.031(3) 0.513 2.025(3) 0.521

V–O(5)

^ix

2.031(3) 0.513 2.022(3) 0.526

V–O(1)

^x

2.342(3) 0.221 2.349(3) 0.217

C1.974D P

s ¼4.20 C1.974D P s ¼4.20

P–O(3)

^x

1.513(3) 1.325 1.515(3) 1.317

P–O(4)

^vii

1.521(3) 1.296 1.518(3) 1.307

P–O(5)

^viii

1.552(3) 1.192 1.552(3) 1.192

P–O(2)

ⁱⁱⁱ

1.559(3) 1.170 1.557(3) 1.176

C1.536D P

s ¼4.98 C1.536D P s ¼4.99

Angle (

) Angle (

)

O(5)

ⁱⁱ

–M–O(2)

ⁱⁱⁱ

77.0(1) 77.1(1)

O(5)

ⁱⁱ

–M–O(1)

^vi

79.2(1) 80.1(1)

O(2)

ⁱⁱⁱ

–M–O(1)

^vi

79.4(1) 79.8(1)

O(4)

^vii

–P–O(5)

^viii

106.5(2) 106.4(2)

O(2)

ⁱⁱⁱ

–P–O(5)

^viii

107.4(1) 107.4(1)

O(3)

^x

–P–O(5)

^viii

109.6(2) 109.4(2)

O(2)

ⁱⁱⁱ

–P–O(3)

^x

109.8(2) 110.0(2)

O(2)

ⁱⁱⁱ

–P–O(4)

^vii

111.3(2) 111.6(2)

O(3)

^x

–P–O(4)

^vii

112.1(2) 111.9(1)

C109.5D C109.5D

O(1)

^x

–V–O(5)

^ix

73.9(1) 73.2(1)

O(1)

^x

–V–O(2)

^vii

74.8(1) 73.7(1)

O(2)

^vii

–V–O(5)

^ix

78.5(1) 77.9(1)

O(1)

^x

–V–O(3)

^viii

83.0(1) 83.1(1)

O(1)

^x

–V–O(4)

^vii

84.8(1) 85.3(1)

O(3)

^viii

–V–O(5)

^ix

89.9(1) 90.7(1)

O(2)

^vii

–V–O(4)

^vii

91.7(1) 91.4(1)

O(3)

^viii

–V–O(4)

^vii

92.0(1) 91.8(1)

O(1)–V–O(5)

^ix

97.9(1) 97.7(1)

O(1)–V–O(2)

^vii

98.0(1) 99.2(1)

O(1)–V–O(4)

^vii

102.8(1) 103.2(1)

O(1)–V–O(3)

^viii

103.3(1) 102.9(1)

O(2)

^vii

–V–O(3)

^viii

157.0(1) 156.3(1)

O(4)

^vii

–V–O(5)

^ix

158.2(1) 157.9(1)

O(1)–V–O(1)

^x

169.9(1) 169.3(1)

Symmetry transformations used to generate equivalent atoms:

ⁱ

x þ1, y þ 1, z;

ii

x 1, y 1, z;

ⁱⁱⁱ

x þ1, y, z þ 1;

^iv

x 1, y, z 1;

^v

x 1, y, z;

^vi

xþ 1, y,z;

vii

x, y þ 1/2, z 1/2;

^viii

x þ1, y 1/2, z þ1/2;

^ix

x þ 2, y 1/2, z þ 1/2;

x

x, y þ 1/2, z þ 1/2.

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634 631

2.026(1) Å for Ni (average 1.976 Å). The shortest V–V distance is 3.751 Å for M ¼ Co and 3.746 for M ¼ Ni.

4.2.2. MO

6

polyhedra (M ¼ Co, Ni)

In the triangular based antiprism M–O distances are ranging from 2.058(2) to 2.095(2) Å (average value: 2.078 Å) for Co, and from 2.031(1) Å to 2.062(1) Å (average value: 2.047 Å) for Ni. Here also average values are in good agreement with the sum of ionic

radii of O

^2þ

and M

^2þ

. Co–Co and Ni–Ni distances are 5.210 Å and 5.191 Å respectively. Like in a phase, connection between MO

₆

polyhedra and VO

6

octahedra by faces leads to short M–V distances, 2.913 Å for Co–V and 2.898 Å for Ni–V. Investigations on possible induced magnetic interactions have been undertaken.

4.2.3. PO

4

tetrahedra

Like in the a -form [PO

₄

] tetrahedra share oxygen atoms with four [VO

6

] octahedral groups and connects three different chains.–V–O(1)–V–O(1)–V–. They are rather regular with O–P–

O angles varying from 106.3(1)

to 111.6(1)

for Co, and from

Fig. 2. V–O–V–O– chain and PO

4

tetrahedra in M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni).

Table 6

Bond distances (Å), bond valences (BV) and angles (

) for b -M(VO)

2

(PO

4

)

2

.

M ¼ Co M ¼ Ni

Distance (Å) BV Distance (Å) BV

2 M–O(5)

ⁱ

2.058(2) 0.372 2.031(1) 0.361 2 M–O(2)

^ii,iii

2.080(2) 0.350 2.062(1) 0.332 2 M–O(1)

^iv,v

2.095(2) 0.336 2.049(1) 0.344

C2.078D P

s ¼2.12 C2.047D P s ¼2.07

V–O(1) 1.633(2) 1.508 1.636(1) 1.492

V–O(3)

^vi

1.903(2) 0.725 1.900(1) 0.731

V–O(4)

^v

1.914(2) 0.704 1.908(1) 0.715

V–O(5)

^vii

2.021(2) 0.527 2.014(1) 0.537

V–O(2)

^viii

2.031(2) 0.513 2.026(1) 0.520

V–O(1)

^vi

2.362(2) 0.210 2.372(1) 0.204

C1.977D P

s ¼4.19 C1.976D P s ¼4.20

P–O(4) 1.510(2) 1.335 1.511(1) 1.332

P–O(3) 1.513(2) 1.325 1.513(1) 1.325

P–O(5)

^viii

1.558(2) 1.173 1.556(1) 1.179

P–O(2)

^v

1.561(2) 1.167 1.561(1) 1.163

C1.536D P

s ¼5.00 C1.535D P s ¼5.00

Angle (

) Angle (

)

O(2)

ⁱⁱ

–M–O(5) 77.1(1) 77.2(1)

O(1)

^iv

–M–O(5) 79.0(1) 80.0(1)

O(1)

^iv

–M–(O(2)

ⁱⁱ

79.9(1) 80.5(1)

O(3)–P–O(5)

^viii

106.3(1) 106.5 (1)

O(2)

^v

–P–O(5)

^viii

106.9(1) 106.7(1)

O(2)

^v

–P–O(4) 110.0(1) 109.9(1)

O(4)–P–O(5)

^viii

110.3(1) 110.4(1)

O(3)–P–O(4) 111.5(1) 111.6(1)

O(2)

^v

–P–O(3) 111.6(1) 111.6(1)

C109.4D C109.4D

O(1)

^vi

–V–O(5)

^vii

73.7(1) 72.9(1)

O(1)

^vi

–V–O(2)

^viii

74.8(1) 73.9(1)

O(2)

^viii

–V–O(5)

^vii

79.1(1) 78.4(1)

O(1)

^vi

–V–O(4)

^v

82.7(1) 82.7(1)

O(1)

^vi

–V–O(3)

^vi

84.0(1) 84.7(1)

O(4)

^v

–V–O(5)

^vii

89.8(1) 90.6(1)

O(3)

^vi

–V–O(4)

^v

90.7(1) 90.5(1)

O(2)

^viii

–V–O(3)

^vi

92.0(1) 91.8(1)

O(1)–V–O(5)

^vii

98.2(1) 97.8(1)

O(1)–V–O(2)

^viii

98.9(1) 100.0(1)

O(1)–V–O(4)

^v

102.7(1) 102.5(1)

O(1)–V–O(3)

^vi

103.7(1) 104.2(1)

O(2)

^viii

–V–O(4)

^v

156.9(1) 156.1(1)

O(3)

^vi

–V–O(5)

^vii

157.4(1) 157.3(1)

O(1)–V–O(1)

^vi

170.4(1) 169.6(1)

Symmetry transformations used to generate equivalent atoms:

ⁱ

x, y, z;

ⁱⁱ

x 1, y, z;

ⁱⁱⁱ

x þ1, y, z;

^iv

x 1, y, z 1;

^v

xþ 1, y, z þ 1;

^vi

x, y þ1/2, z 1/2 ;

^vii

x þ 1, y þ 1/2, z þ 1/2;

^viii

x, y þ 1/2, z þ 1/2.

Fig. 3. Structural filiations for a -M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni). a. Li(TiO)(PO

4

) ortho-

rhombic type. b. LiNi

0.50

(TiO)

2

(PO

4

)

2

monoclinic type. c. a -M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni)

type.

106.5(1)

to 111.6(1)

for Ni. P–O bond distances are ranging between 1.510(2) and 1.561(2) Å for Co and between 1.511(1) and 1.561(1) Å for Ni.

5. Structural filiations

5.1. The common structural unit: (A,B) sheet

Structural filiations for a -M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni) and between a and b varieties can be done based on ‘‘(A,B) sheets’’ as common structural unit (Fig. 5). One sheet is constituted of two planes (A and B) parallel to (1 0 0) of infinite chains of [VO

6

] octahedra (Fig. 2) linked together along b axis by [PO

₄

] tetrahedra.

Divalent cations M

^2þ

(M ¼ Co, Ni) are inserted inside these sheets, in trigonal antiprisms of the (2a) site.

5.2. Structural filiations for a -M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni)

The three dimensional framework of the a phase can be described by a succession along a of (A,B) sheets with M

^2þ

cations (Fig. 3c). The (2b) sites between two successive (A,B) sheets are empty. This description can be correlated with the Li(TiO)(PO

4

) (orthorhombic Pnma) [14] and LiNi

0.50

(TiO)

2

(PO

4

)

2

(monoclinic P2

1

/c) [7] types.

In Li(TiO)(PO

₄

) (2a) and (2b) sites are equivalent, corresponding to the (4a) site of the orthorhombic Pnma space group. This site is fully occupied by Li

^þ

cations (Fig. 3a).

For LiNi

_0.50

(TiO)

₂

(PO

₄

)

₂

the monoclinic deformation of the cell (P2

1

/c) is quite similar to that we observed for the a phase: the (2a) site inside the (A,B) sheet is fully occupied by Li

^þ

ions while the (2b) site between two sheets is partially occupied (50%) by Ni

^2þ

ions (Fig. 3b).

5.3. Structural filiations between a and b varieties

In the b phase we can define a (A

⁰

,B

⁰

) sheet deduced from the (A,B) one by a c/2 translation. The three dimensional framework can then be described by a succession of . (A,B)(A

⁰

,B

⁰

)(A,B)(A

⁰

,B

⁰

) . sheets along a parameter, with a monoclinic deformation of Ni(TiO)

₂

(PO

₄

)

₂

type [6] (Fig. 4). Here also the (2b) site between two successive sheets is empty.

In the b -cell, taking into account the origin translation O a O b ¼ (0,1/2,1/2) and for atoms in the (A,B) sheet corresponding to x ¼ 0 (Fig. 5), the atomic coordinate relations are the following:

x _b ¼ x a ; y _b ¼ y a þ 1=2; z _b ¼ x a =2 z a þ 1=2

6. Thermal study of the a -M(VO)

2

(PO

4

)

2

4 b -M(VO)

2

(PO

4

)

2

(M [ Co, Ni) transition

- As indicated in Section 2.1 obtaining of a or b phase depends on the heating temperature of the reagents (VO

2

and M(PO

3

)

2

) used for the synthesis.

- If the silica tube is heated at 700

C for 40 h, the a phase is obtained, and if it is heated at 900

C for 40 h, it is the b phase.

Mixture of the two phases is observed when cooling from intermediate temperatures.

- When the a phase undergoes a thermal treatment in silica tube at 900

C for 40 h, the a phase is conserved.

6.1. DTA study

DTA studies have been done starting with a or b phase for M(VO)

2

(PO

4

)

2

with M ¼ Co or Ni in sealed platinum tube under argon. Starting with the a phase, no phase transition effect is observed until melting (1050

C for Co and 1080

C for Ni), then cooling to room temperature re-crystallization leads to the b phase.

Fig. 6 illustrate results for M ¼ Ni. Starting with the b phase, no phase transition is observed until melting (w1080

C), and b phase is kept after re-crystallization.

6.2. High-temperature X-ray diffraction study

An X-ray diffraction study has been done for a -Ni(VO)

₂

(PO

₄

)

₂

with He gas-flow in the HTK16 furnace. Several data collections have been carried out during heating (25, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1020, 1040, .

C) with the following conditions: angular range 10–80

(2 q ) ; steps of 0.02

(2 q ); counting time of 5 s by step; slope of 2

C/min and a holding time of 30 min for each level. We particularly followed the evolution of the dif- fracted peak w 22

(2 q ), which would disappear if a / b transition occurs (Fig. 1). Up to 1100

C pattern’s evolution shows dilatation of the a -cell, lowering of intensities, but no transition. Fig. 7 shows cell parameters and volume dependence with increasing temperature.

Fig. 4. b -M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni) type.

Fig. 5. (A,B) sheet as common structural unit for a and b -M(VO)

2

(PO

4

)

2

(M ¼ Co, Ni).

Fig. 6. DTA study starting with a -Ni(VO)

2

(PO

4

)

2

.

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634 633

At 1120

C a strong relative decrease of the diffracted peak w 22

(2 q ) is observed but with the beginning of melting. At 1140

C melting is done. After cooling down to room temperature, powder on Pt-filament is recovered and X-ray diffraction analysis confirms it is the b phase. These observations agree with the DTA results. The higher values obtained for melting temperatures in the X-ray study can be explained by systematic error between temperature control of the HTK16 furnace and real surface temperature of the sample.

7. Conclusion

New vanadium oxyphosphates M

^II

(V

^IV

O)

₂

(PO

₄

)

₂

(M

^II

¼ Co, Ni) have been obtained. According to synthesis temperature with starting reagents, two phases named a and b have been prepared.

The four crystal structures have been determined in monoclinic P2

1

/c space group using X-ray single crystal diffraction data.

Structure of the a phase can be correlated with Li(TiO)(PO

4

) and LiNi

0.50

(TiO)

2

(PO

4

)

2

types. Structure of the b phase is derived from

the Ni(TiO)

2

(PO

4

)

2

-type we have found for several titanium oxy- phosphates M

^II

(TiO)

₂

(PO

₄

)

₂

. Structural filiation has been discussed based on a common structural sheet built up of parallel infinite chains of tilted corner-sharing [VO

6

] octahedra, cross-linked by corner-sharing [PO

₄

] tetrahedra. Divalent cations M

^2þ

(M ¼ Co, Ni) are inserted in trigonal antiprism sites of these sheets.

The thermal study, by DTA or by X-ray diffraction, shows that the a phase obtained from the mixture of starting reagents, at w 700

C, is stable until melting. Whatever the starting phase is ( a or b ), after melting the b phase is always obtained.

Acknowledgements

This work was supported by the ‘‘Programme International de Coope´ration Scientifique’’ (PICS) of CNRST/Morocco-CNRS/France.

The authors thank E. Lebraud for high temperature X-rays diffrac- tion studies, P. Dagault and D. Denux for DTA study.

References

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[10] J. Rodriguez-Carvajal, FULLPROF: a program for Rietveld refinement and pattern matching analysis Toulouse, France, in: Collected Abstract of Powder Diffraction Meeting (1990), p. 127.

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[12] G.M. Sheldrick, SHELXS-97 and SHELXL-97, University of Go¨ttingen, Germany, 1997.

[13] I.D. Brown, D. Altermatt, Acta Crystallogr., Sect. B 41 (1985) 244.

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Neorg. Khim. 36 (1991) 2766.

0 200 400 600 800 1000

6,2 6,4 6,6 6,8 7,0 7,2 7,4 7,6

90,0 90,2 90,4 338 340 342 344 346 b

c

V a, b, c (Å) β

T (°C) a

V (Å 3 ) β (°)

Fig. 7. a -Ni(VO)

2

(PO

4

)

2

cell parameters and volume dependence with increasing temperature.

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